pnutty

Absolutely, my local dealer has a adjustable gas block in stock as well so might go that route just because I am impatient haha . I happened to stumble upon these modified gas tubes that have the gas hole drilled to the size you want, I wonder if these would be a better alternative to an adjustable gas block as I doubt these could fail like the adjustable gas block you described at your match. The price seems expensive for what you get though... https://blackrivertactical.com/WP/BRT-EZTUNE-Gas-Tube-Configurable-Rifle-p334967178

Thank you for putting all that information into something I can understand and read up on. I also looked at faxon’s website and they now sell their adjustable gas blocks in .625” .750” and .875” diameters so I will pick up the size I need. It is a pretty ingenious design keeping the screw out of the gas port and adjusting it by moving the gas tube. I will report back once everything comes in and I get out to test it again. Again thank you for figuring this out for me, I would have been running around in circles trying to get it to run correctly.

Thanks! Would you have any suggestions on gas block brand? I just keep hearing the adjustment screws seize in those things after awhile. I do have to ask how journal diameter affects the gas port size, I would figure a hole is a hole that moves the same amount of gas no matter the journal size but you know much more than I. You have me interested in how all this works. One last question is would a heavier buffer have the same outcome? I know the higher the weight of the buffer, the more it delays the bolt movement which would let chamber pressures lower and let the case shrink for better extraction. I am going to purchase the gas block, like I said I am very interested in how this entire system works. Thanks again for all your help and troubleshooting.

I do have a .875 journal but I doubt that would make that much of a difference, any other ideas to check or do I get an adjustable gas block or extra heavy buffer?

Update to the thread, did ammo testing today and have some numbers. I also made up a free float jig for my magnetospeed so now I can zero without effecting barrel harmonics or zero. Hornady 140g eldm ; avg-2596 sd-7 es-15 S&B 140g fmj; avg-2566 sd-16 es-32 Aguila 129g SP; avg-2647 sd-12 es-29 Reloads 140g eldm with40g staball; avg-2575 sd-7 es-17 Barrel was cleaned and all residual oil wiped out of barrel before testing. I also soaked the bcg in oil and ran it very wet but still getting swipes and extractor marks on the case rim.

Very informative, sounds like I would need a heavier buffer but only 1 or 2 options after a H3

If everything goes well, I will hopefully be testing tomorrow. Before I installed the barrel I measured the gas port and a #47 (.0785”) drill fits perfectly, a #46 drill would not fit (.081”) so that gives you the port size at least. I recall reading (possibly on here) that the 6.5CM runs higher port pressures than the 308win for the same barrel length and gas system, I wonder if that would have anything to do with it. I did fire the same ammo except the Aguila and the same thing happened with the DPMS bolt but I was getting pierced primers so that is why I swapped. I figured if I had issues with 4 different ammo types then something has to be causing it in the gun. I was thinking maybe the leade or throat was very short and jamming the bullets in the lands but I colored in the bullet with a sharpie and loaded a few rounds and ejected them. No marks were on the bullets at all so there goes that theory.

No issues with chambering and extracting a unfired round, I did rent headspace gauges when I first built it and no issues but I did swap the original dpms large firing pin bolt with a Kak small firing pin dual extractor bolt. The locking lugs thickness and the bolt face depth are identical on both so I doubt that would cause an issue.

I did, the buffer is 5.5oz so that should be right in spec for an H3 Alright, thought I had all the important info but here is more; The bolt does have the dual ejector spring setup with the oring setup that way from KAK. The buffer does weigh 5.5oz so that is in spec for an H3 and the EA1095 spring was purchased from everygunpart which Armalite uses as their oem so I highly doubt its a knockoff. The bolt locks back properly on the boltface everytime so I doubt that it is undergassed. I am going to follow 98Z5V’s advice and chrono the ammo im using and hopefully will have some answers after that.

I do have a magnetospeed sporter so when I get to the range again ill report back. Just curious if the ammo isnt an issue, could the gun parts be out of spec and that would cause this?

I took the best pictures I could of the case heads, the marks dont like to show up for the camera though. You can see the spots where both ejectors are making an imprint but one ejector is much more pronounced than the other one, why that is I have no idea. I also took a picture of the extractor indent on the case if that would help as well. S&B had the least amount of swipe and I could not get that to focus at all on the camera.

Ok sounds reasonable, I usually put some oil on my fingertip and smear it all over the outside as a film coat but ill do what you said 👍

Yes they are both rounded, and I agree with what you are saying about it not shrinking fast enough. The brass is randomized in the pic, I shot Aguila, S&B, Hornady then reloads in that order. There are 2 ejector marks on the cases but only one of the ejectors is showing up much more pronounced than the other one. On the S&B case the marks are on the “cr” and “oo” in Creedmoor but it’s hard to tell from the picture, I can try to get better pictures for you if needed. But I do not know why one ejector mark is showing up much more than the other one…

Hi again; I’m having a few more issues with my build, specs are: Aero m5 receiver set BA 6.5CM 20” barrel rifle length with .0785 gas port with ar15 rifle gas tube in the middle of cutout Kak 308 bcg dual ejector with small firing pin/hole, oring and 2 ejector springs installed Kak extended buffer tube 7-11/16 deep with H3 buffer and EA1095 spring Cmmg lpk What is happening is that I am getting ejector swipe on 4 different brands of ammo and a scenario where the extracted round doesn’t eject but stays in the receiver and the bolt travels rearward enough to pick up a new round. If I single load a round the bolt locks on an empty mag but the empty round still stays in the receiver. This build has around 100rds through it so nothing should be worn out. Ammo is S&B 140g, Aguila 129g, Hornady 140g and my reloads in the Lapua brass which has 42g of staball that puts its right in the middle of the reload book spec. My unprofessional opinion is that the cases are still expanded while trying to extract and that’s why they aren’t exiting the upper, I doubt it is undergassed because the bolt goes rearward enough to pick up a new round or lock backnon an empty mag.

This is what scares me about purchasing a muzzle brake, I read that this is sort of thing can happen so I do all my development with just a muzzle protector installed. It’s unfortunate you spend 3 figures on a muzzle device just to get pie plate grouping.

I have a few questions for you, if you dont mind. Is it worth it to use a muzzle brake on a hunting rifle, and regarding the cantilever mount, is that a Leupold mark ar mount? If so how do you like it, I have seen them go for a pretty good price so I was suspicious about the quality.

Nice elk 👍 Could you share the build details?

I visited my lgs and he had one more aero m5 upper hidden away that he let me purchase. I already built an upper just for steel targets so I thought maybe I could build this upper for hunting whitetails and the occasional coyote that gets in the way. Has anyone successfully used a large ar platform for hunting and if so, how is it set up? I was thinking 7mm-08 for the caliber with a 16”-18” lightweight barrel, but im open to any and all suggestions!

I did put a box of S&B 140g fmj through it and no pressure signs I saw except some but not all of the primers are flattened. One thing I noticed is compared to my bolt 6.5CM (I know apples to oranges) is that the firing pin indent is much deeper, I don’t know what the spec is for large frame ar’s but I measured using a depth micrometer and from front of bolt to bolt face is .125” and front to the tip of the firing pin when engaged is .076” so that’s a .049” protrusion. Possibly this could be an issue as well? One last thing is that I do have srp 6.5 brass for my other gun as well, would that have any advantages to use with a gas gun?

I will, I ran this load in a bolt gun with no problems but that’s a different animal.

It was a handload, 40.2g of imr4451 over 140g sst. Primers are cci #34

Hello everyone, I finally taken my first large frame 6.5cm build to the range and everything worked great. Put 70 rounds through it and no issues functioning but I was puncturing primers. (Indoor ranges aren't ideal but it was good enough for a function test and scope zero) When I got home and started to clean it, I couldn't remove the cotter pin at all. I had to clamp onto the round part of the cotter pin with the corner of the vise and yank it out that way. Once I got it out I found the pin was bent to hell, I also noticed a few pieces of primers that came out when I finally got the bolt apart. (Primer pieces are at very top of the photo) From my research, the primer issue is because of excessive clearance between the firing pin and hole. I measured my firing pin hole at .082” and the firing pin tip at .077”, I found that cmmg makes a .08” firing pin for their mk3 model rifles. My questions are if the cmmg firing pin will work in my dpms bcg and if the primer cups getting lodged in the bcg are the reason the cotter pin bent. Specs are m5 receiver sets cmmg lpk armalite recoil setup dpms bcg ba 6.5cm 20” barrel rifle gas length

Absolutely, I was stalking this place for awhile before making an account. I wanted the entire Armalite recoil setup but the tube and buffer seem to not be available anymore.

Hello from another ban state dweller as well ,as I am in the peoples republic of NY. Just curious how you made yours "compliant" as in Ny, the most common thing to do is pin the mag which is what I did.

Ended up getting the barrel installed, gas tube looks is right in the middle of the cam cutout and I used the bore scope to make sure the gas block was aligned. Just waiting on a few more parts to finish it up.